1. Field of the Invention
The present invention relates to a new material used in manufacturing surface mounting devices and a process of making said material. Particularly, the invention pertains to a material for making high performance hyper frequency multi layer chip inductors, which can be sintered at a low temperature.
2. Description of the Related Art
Multilayer chip inductors (xe2x80x9cMLCIxe2x80x9d), also known as multilayer ferrite inductors (xe2x80x9cMLFIxe2x80x9d), are one of the most important surface mounting devices (xe2x80x9cSMDSxe2x80x9d). The key technical aspect is the co-firing between ferrite and an internal conducting material, such as silver (Ag, chosen for its good conductivity and lower costs). It requires the ferrite to have a low sintering temperature and a high initial permeability. The main materials currently available for manufacturing SMDS are Nixe2x80x94Znxe2x80x94Cu ferrite sintered at 850-900xc2x0 C. and low dielectric constant ceramics. The Nixe2x80x94Znxe2x80x94Cu ferrite, has a resonant frequencies below 200 MHz, which may reach 300 MHz if Zn is excluded from the ferrite. Thus, Zixe2x80x94Znxe2x80x94Cu ferrite is not suitable for making MLCIs operating in hyper frequencies (300-1000 MHz). On the other hand, low dielectric constant ceramics, with a permeability equal to 1, has been used for making MLCIs operating in ultra high frequencies ( greater than 800 MHz), where high inductance is not required. Therefore, there is no ideal material for making MLCI operating in the frequency region from 300-800 MHz.
In 1950xe2x80x2s, Philip developed Co2Z hexaferrite (Ba3CozFe24O41). It generally has a high sintering temperature (xcx9c1300xc2x0 C.). Recently, it was found that using a chemical synthetic method with citrate precursor could be used to prepare Co2Z hexaferrite with a lower formation temperature (xcx9c1150xc2x0 C.). The chemical synthetic method also improves compositional homogeneity, which is particularly important for doping elements. However, to the applicants"" knowledge, no one has taught or suggested any CO2Z hexaferrite which can be sintered at a temperature below 900xc2x0 C.
The present invention discloses a new material composition used in making high performance hyper frequency multiplayer chip inductors (xe2x80x9chfMLCIxe2x80x9d) and a process of preparing said composition. The composition of the present invention can be sintered under 900xc2x0 C. so that the hfMLCI made from this material is suitable for the frequency region of 300-1000 MHz with better inductance and reliability, and can be manufactured with relative lower costs.
The composition of the present invention has a major component and minor component. The major component accounts for 88-99% by weight and the minor component accounts for 1-12% by weight. The major component is Z-type planar hexaferrite having a general formula of Ba3Co2-x-yZnxCuyMnzFe24-z-wO41-3/2(z+w), wherein 0xe2x89xa6xxe2x89xa61.0, 0xe2x89xa6yxe2x89xa60.8, 0xe2x89xa6zxe2x89xa61.0 and 0xe2x89xa6wxe2x89xa61.0. The minor component acts as a sintering aid, having a formulation of aBi2O3+bV2O5+cPbO+dB2O3+eLiF+fCaF2, where 0xe2x89xa6axe2x89xa61, 0xe2x89xa6bxe2x89xa61, 0xe2x89xa6cxe2x89xa61, 0xe2x89xa6dxe2x89xa61, 0xe2x89xa6exe2x89xa61, 0xe2x89xa6fxe2x89xa61.
The composition of the present invention can be prepared by following the steps described below:
(1) weigh out each salt to prepare a solution contain Fe3+, Co2+, Ba2+, Zn2+, Cu2+, Mn2+ in predetermined molar ratios, with Fe3+xe2x80x2s concentration being 0.4-5 M;
(2) dissolve Fe3+ salt in water and add NH4OH of an equal molar number to precipitate Fe(OH)3, collect Fe(OH)3 by filtration and wash it with water, and then dissolve the freshly prepared Fe(OH)3 in a citric acid solution (Fe3+/citric acid mole ratio should be 1-2) at 60-80xc2x0 C. with stirring until the solution becomes transparent;
(3) add all the remaining salts weighed out in step (1), that is, all the salts except Fe3+, to the solution prepared in step (2), add NH4OH to adjust the pH to 6.0-8.0, and wait for about 2 hours until the solution becomes sol;
(4) incubate the sol obtained in step (3) in an oven at 130-150xc2x0 C. until it dries up;
(5) heat-treat the dried-up gel at 900-1250xc2x0 C. for 2-6 hours to obtain Z-type hexaferrite powder;
(6) mix the Z-type hexaferrite powder with the sintering aids, i.e., the minor component of a predetermined formulation, grind the mixture in water or alcohol in a ball mill;
(7) dry up the ground mixture at 80-120xc2x0 C., sieved and press into pellets; and
(8) sinter the pellets at 900-1250xc2x0 C. to obtain the composition of the present invention.
It is also contemplated that the above described procedure can be varied without departing the present invention. For example, although the above procedure stated that Fe3+ was first precipitated with NH4OH before mixing with the other salts weighed out in step (1), it is possible that Fe3+ is first mixed all the other salts in a solution and then added with citric acid and NH4OH, with the resulting solution being used to continue in steps (4)-(8). Another possible variation lies in step (6). While in the above specified procedure the Z-type hexaferrite powder obtained in step (5) is first mixed with the sintering aids before grinding in a ball mill, it is entirely possible that the Z-type hexaferrite is ground alone and dispersed into ethylene glycol and then mixed with a solution of the sintering aids which, after adjusting the pH, forms a hydroxide coating on the surface of the Z-type hexaferrite particles.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the description and drawings are provided solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the claims.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.